1. Field of the Invention
The present invention relates to a solid ionic conduction material reinforced by a cellulosic material or a chitin, which can be used as an electrolyte of a battery, of a fuel cell, of a supercapacitor or of an electrochromic device, or as a component of a composite electrode.
2. Description of the Related Art
In a generator in which the electrolyte is a film of a material consisting of a readily dissociable salt dissolved in a non-plasticized polymer solvent, it is possible to reduce the internal resistance of the generator either by increasing the ionic conductivity of the polymer electrolyte or by reducing the thickness of the electrolyte film. The two aforementioned methods are in conflict because the ionic conductivity dictates the choice of functional polymer matrices which make it possible to dissolve the salt, solvate and mobilize the ions. The polymer matrices must therefore have the lowest possible degree of crystallinity in the working temperature range. However, reducing the degree of crystallinity of a polymer generally entails reducing the mechanical strength. In a polymer electrolyte, the macromolecular chains provide the mobility of the anions and cations. The mobility of the ionic species increases when the degree of crystallinity and the glass transition temperature decrease. It would therefore seem difficult to optimize the properties of a polymer electrolyte.
Various solutions have been envisaged in order to improve the mechanical strength of the polymer matrix. The degree of crystallinity of the polymer solvent has been reduced, in particular, by using comonomers other than ethylene oxide [F. Alloin et al., Electrochimica Acta, 40, 2269 (1995)]. A crosslinked polymer matrix has also been used [F. Alloin et al., J. of Electrochem. Soc., 141, 7, 1915 (1994)]. Fillers have likewise been introduced into the polymer matrix in the form of a powder or fibers (glass, carbon, aramid, alumina, silica fibers). Alumina and silica offer an improvement only when oligomers are used. Glass, carbon or aramid fibers improve the mechanical strength, but do not allow the production of thin films.
The use of cellulose in order to reinforce polymers is furthermore known.
For example, WO 95/23824 describes composite materials consisting of cellulose microfibrils dispersed in a polymer matrix. An aqueous dispersion of microfibrils is used in order to prepare a reinforced latex. The microfibrils fulfill the role of a thickener in the aqueous dispersions. These aqueous dispersions are used for paints, inks, varnishes, compositions for aqueous adhesives and ground surface coverings.
U.S. Pat. No. 5,964,983 describes the preparation of cellulose microfibrils from parenchyma (sugar beet, fruit and vegetable pulp) or wood. One of the properties mentioned for the microfibrils is their ability to form films and reinforce other materials, in particular latices or thermoplastic compounds or cellulose acetate. These microfibrils are used as a thickener for food products or for cosmetic products, as an agent improving the opacity and uniformity of paper, or as an agent improving the mechanical strength of paper.
FR 2,743,371 describes a polymer matrix reinforced by cellulose microfibrils coated with polypyrrole. Latices reinforced by dispersing the coated microfibrils in an aqueous medium are used in order to prepare composite films which are resistive or capacitive, depending on the proportion of coated cellulose microfibrils. The coated cellulose microfibrils furthermore exhibit an electronic conductivity which is inherent to polypyrrole, and this property is imparted to the composite materials which contain them.
WO 97/12917 describes the use of cellulose microfibrils as a reinforcing filler or as a structuring filler of a composite material, in particular for polymer matrices such as cellulose esters (for example cellulose acetate) and biodegradable polymers such as polyhydroxybutyrate and polyhydroxyvalerate.
EP 859 011 describes a method for obtaining cationic cellulose microfibrils, or their soluble derivatives, by reaction with a quaternary ammonium salt. The compounds obtained in this way can be used as a thickener in the field of paints, cosmetics, in the chemical industry, as a film-forming agent in papermaking, or as a flocculant in water treatment.
FR 2,784,107 describes the preparation of surface-modified cellulose microfibrils and their use as a texturing agent or reinforcing filler. The microfibrils are used in order to reinforce thermoplastic materials, thermoset materials, or crosslinked or non-crosslinked elastomers. The reinforced materials can be used for the production of a battery separator.